The present invention relates to a process and apparatus for sealing paper sheets along at least one edge thereof and particularly to the closing of envelopes, such as so-called self-enveloping forms without using to adhesive substances.
In the following description, self-enveloping forms will be disclosed as an example without this being construed as a limitation of the scope of the invention.
As it has been known in recent years the so-called self-enveloping forms have acquired widespread popularity such forms have been especially used for printing and mailing invoices, bills, etc., particularly from companies and suppliers of public utility services (electricity, gas, telephone, tap water), which are compelled to issue at very close times (actually every two months) bills or invoices for a great number of customers, such bills are to be mailed and delivered by the mail service (or equivalent service), and in order to take advantage of a discounted tariff with respect to the normal rate, the envelope must have an open side for postal inspection.
The self-enveloping form has permitted meet such a requirement since it can be supplied to the billing and printing centers as a continuous form strip and, once it has been printed and completed with the address of the customer, it is folded to form an envelope.
Such an envelope thus has one side thereof being naturally closed in correspondence to the final folding of the sheet which has already been folded in two or three parts at parallel folding lines, and the envelope must be then sealed along at least two of the three remaining edges depending on whether one open edge for postal inspection must be left.
According to a known and used technique such closing and sealing process has been carried out by coating a sheet which has not been yet folded but was provided with weakening lines for separating edges to be torn off and/or portions of the sheet, with an adhesive (either reversible or unreversible, depending on whether more than one operation of opening and closure of the "envelope" is required) before the processing thereof at the printing and billing center.
In the most widespread types of self-enveloping forms that the adhesive had to be applied along edge stripes of the still blank form or sheet on both sides of the sheet and the stripes were attached to mate with each other after several folding operations.
This fact has been a not negligible drawback and problem, as discussed in the preamble of the Italian patent No. 1,169,138 filed on 24th Nov. 1983 in the name of the inventors of the present invention.
This Italian patent discloses a solution of the technical problem of closing envelopes and particularly self-enveloping forms. The solution involves knurling the edges to be sealed, preferably by action of two opposed knurling rollers between which the superimposed and mating edges to be sealed are passed.
The pressure applied between the two rollers causes surface portions of the paper to be mutually deformed thus sealing together the two superimposed edges of the paper.
Obviously such a solution is useful only for the cases of sealing, sealing which can not be opened without being destroyed. The success of the sealing operation depends, however, on the careful control of the mechanical conditions under which it is carried out and on the paper having constant characteristics owing to the fact that, as already mentioned, the sealing is ensured by the simultaneous deformation of mating portions of the two sheets or edges of the paper sheet undergoing the action of the relieved and depressed portions of the two knurling rollers.
It is evident that any improvement in the surface deformation leads to an improved sealing, so that it is made suitable also for the cases to date excluded by the mailing service from the number of uses of this process, for instance the registered mail.